On 21/10/2011 12:57, Razmig K wrote: > Le 21.10.2011 12:26, Ivan Voras a écrit : >> Well, do you know that SIZE in top is virtual memory size, not resident >> size (which is the "RES" column)? You can allocate whatever you want >> from virtual memory, it is not "used" until it's touched. > > Yes, I do. So do you suggest using RES as a better indicator of memory > footprint?
Almost certainly yes. Measuring virtual memory is significantly less important for real-world loads. Some of this is very nicely described here: https://www.varnish-cache.org/trac/wiki/ArchitectNotes . > The program in question processes large 3D images via vtk, and I'd like > to measure its memory usgae with different parameter configurations as > the maximum amount of memory acquired during execution. Since SIZE often > happens to be larger than RES, and increase more during execution, I > thought of using it as an indicator of memory footprint. No; the difference between SIZE and RES is "slack space" - allocated but untouched virtual memory, which is *NOT PRESENT IN RAM*. You can verify this yourself: make a small C program and allocate twice the physical memory (+swap) you have on the machine (try terabytes on a 64-bit machine), and it will succeed. If you look at this program in top, it should (barring some optimizations) show you that SIZE is huge, but RES is a couple of MB, basically like you didn't allocate anything at all. Now, it is a whole other thing if you try to actually *use* this memory you've allocated. Here's a random link on the topic from Google: http://opsmonkey.blogspot.com/2007/01/linux-memory-overcommit.html . Unfortunately, the phrase "memory overcommit" has been hijacked by the virtualization environment to mean the same thing but relating to the memory in virtual machines.
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